2. 6. Conclusion and future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
According to the history perspectives, AC power network has
been the standard choice for commercial energy systems to power
the incandescent lamps in our residences and motors in our
factories since the late of 19th century. The easy to transform AC
voltage into different levels for various applications, the capability
to transmit power over a long distance and its inherent character-
istic from the fossil energy driven rotating machine, made the AC
power network to become the main choice. To this point, the AC
power networks have existed for more than one century ago along
with the AC loads dominated in the market [1]. However, high
energy costs together with limited fund to construct new large
power plants and long distance transmission lines are some of the
constraints to meet the growing energy demands. Moreover, the
global warming, aging of the current power system infrastruc-
tures, increased awareness of limited energy generation resources,
higher energy consumption standards and the increased number
of DC loads due to advance in power electronics reveal that
modernization of the current energy system is inevitable [2].
In the last two decades, modern solutions such as renewable
based DG units, energy storage systems (ESSs), flexible AC trans-
mission systems (FACTS), active demand management (ADM), AC
microgrids and advanced control strategies based on information
and communication technologies (ICTs), have made possible for
energy engineers and researchers to redesign the conventional
power systems. However, not all these redesigning procedures are
accomplished until now, as more researches are needed to make
such advanced systems a reality. In that case, the designed and
implemented AC microgrid systems utilize the existing AC power
system standards such as frequency, voltage levels and principles
of protection for their operations [3,4]. On the other hand, the
introduction of grid control using the mercury vapor rectifier
around 1903 gave rise to electronic devices application in high
voltage direct current (HVDC) transmission in 1928. This was due
to their remarkable ability for rectification and inversion of DC
power [1,5]. Since then, the DC power systems had been restricted
only to some special applications such as telecommunication
systems, shipboards, tractions, HVDC transmission systems and
AC power systems for control and protection [6–8]. However, the
development in power electronics technology has raised a number
of DC loads and more power converters for DC voltage transfor-
mation into various levels for different applications [9–11]. Today,
the DC distribution systems (DC microgrid systems) are applied in
avionics, automotive, marine and manufacturing industries for
power distribution [12–14]. The international space station (ISS),
spacecraft, modern aircraft and modern electronics loads such as
computers and servers in data centers, and banks and electronics
factories, require DC power supplies [15–17].
To manage our future energy demands, a more configurable,
flexible, informative and physical energy system in the low voltage
distribution networks if not the whole power network, is manda-
tory [18–20]. Due to this point of view, microgrids (MGs) are
emerging and becoming more attractive structures with integra-
tion of renewable based DG units and ESSs [21,22]. So far, the
DG units include several technologies such as wind turbines
[23–25], photovoltaic (PV) arrays, fuel cells, and micro-turbines
[27] while the ESSs are the batteries, flywheels and super-
capacitors [28–30]. These components are the central ideas in
the microgrid structures and are also regarded as the solution to
the population without access to energy or with poor energy
supply and entitled to effectively reduce the greenhouse gas (GHG)
emissions [31–33]. Furthermore, the DG units provide a relief to
stressed conventional power transmission and distribution infra-
structures [34–36]. Besides, the liberalized electric market with
demands for efficient, reliable and diversified energy resources
constitutes the most important driving forces for high penetration
of DG units into the energy systems.
At present time, most of renewable based DG units directly
produce DC or variable frequency/voltage AC output power and
hence power electronics devices (PEDs) have become the key
elements in order to realize the MGs. Because of their vicinity to
different types of loads (i.e. AC and DC), the DG units such as the
fuel cells and the micro-turbines can be used to provide combined
heat and power (CHP) generation with improved overall efficiency.
With ESSs connected to the DG units such as fuel cells, a
controllable output power to meet the grid codes or load transient
requirements can be produced. The wind turbine and PV array
based DG units are usually controlled with maximum power point
tracking (MPPT) to maximize their energy generation [37–39].
Similarly, the ESSs with wind or PV based DG units can also
produce controllable output power to facilitate the dispatched
power generation and load demand matching [40–42]. Conse-
quently, the MG provides an opportunity to optimize the utiliza-
tion of renewable energy resources with improved overall thermal
and electrical efficiencies by properly locating different DG units
while considering their geographical conditions and the nature of
available loads [43]. Such operating conditions require the micro-
grid systems to have wide-range control systems in order to
perform large number of tasks [44]. For example, the guarantee
to system security, optimal operation, emission reduction and a
seamless transfer algorithm from grid-connected mode to islanded
mode without violating system constraints and regulatory require-
ments are some of the main tasks [45,46]. Also, the MGs for
standalone and grid-tied applications have been considered in the
past as separate cases [47–49]. However, it is important nowadays
to conceive flexible MGs that operate in both modes [50–52].
System protection within the microgrid components [53] and
short-circuit currents limitation can be achieved through a proper
coordination of the DG units [54–56].
This paper presents the latest researcher's literature review in
the field of AC and DC microgrid systems integrated with renew-
able based DG units, ESSs and mixed (AC and DC) loads. The
motivation to carry out this study is the growing interests of
renewable based DG units, ESSs and the continuous changing in
the type of customer's equipments from the dominated AC-type to
DC-type loads. Thus, a survey on microgrid systems with various
DG units configuration in low voltage AC (LVAC) and DC (LVDC)
networks in regard to their current and future applications in
customer equipments and energy market are included. Referring
to the economical, technical and environmental advantages of
renewable energy sources (RESs), a detailed overview between AC
and DC microgrids is provided in relation to their feasibility,
control strategies and protection approaches.
The paper is therefore structured as follows: a description of
the various DG units' configurations and their interconnections
with low voltage AC and DC networks in relation to the power
output is provided in Section 2. Study about the feasibility of
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388
3. microgrid systems is discussed in Section 3 while the control and
energy management strategies in microgrids are presented in
Section 4. Furthermore, the discussion on microgrid system
protection techniques is given in Section 5. Finally, Section 6
draws the conclusion and future trends.
2. Overview of microgrids with DG units
Distributed generation (DG) units as referred to small genera-
tors or decentralized generations can either be used as stand-alone
systems at an isolated area (e.g. rural areas) or utility-connected
systems [57–59]. When used as stand-alone systems, the DG units
are usually operated by individuals to serve small houses such as
single households to large buildings e.g. housing estate or suburb
locality, an academic or public community, an industrial site or
municipal region [60,61]. Larger capacity DG units are managed by
the utility or an independent power producer (IPP). The utilities
use DG units to help them to improve power supply flexibility,
quality and expandability, system stability, optimize distribution
system and reduce transmission and distribution cost. In most
cases, the DG units produce incompatible AC power or DC power
as a result the desired voltage magnitude, frequency and phase
angle are usually obtained through the use of power electronics
interfaces [62–64]. Each DG unit can be connected to the main grid
by using the appropriate power electronics interface. However,
using a single power electronics interface for all DG units leads
to advantages of reduced losses, easier design and control with
reduced cost [65,66].
So far, the DG units based on RESs are generally more sustain-
able (i.e. their energy sources will not perish in the long run) with
little or no environmental damage than their counterparts. Solar
PV arrays [37,66], geothermal and wind [25,26], tidal waves
[28,31], low-head (small) hydro, biomass and biogas [38] together
with hydrogen fuel cells [42] (hydrogen extracted using renewable
sources) are examples of RES based DG units. These DG units can
be aggregated to provide DC input voltage for the DC/AC inverter
for grid-connection or strategically connected to the low voltage
distribution systems (i.e. LVAC or LVDC networks) depending on
their power output to form microgrid structures [67,68]. Alterna-
tively, the microgrid concept enables high penetration of various
DG units and ESSs without the need of restructuring the distribu-
tion system itself. The next Sections 2.1, 2.2 and 2.3 present the
definitions and overviews of the DG units in LVAC network, LVDC
network and microgrid systems with the connection possibilities
of various DG units and loads.
2.1. Definition of LVAC networks
All electricity generating units (e.g. DG units) with AC power
output are directly connected to an AC bus line and then to the
main system via power converters for their stable coupling.
Examples of the DG units that produce the AC output power
include wind turbines, low-head hydro, biogas, tidal and wave
turbines [4]. These are usually directly connected or may need the
AC/DC/AC power converters to enable their stable coupling with
the LVAC networks. In that case, the LVAC network can be
interconnected with the bulk system (utility) through a power
transformer. In addition, the AC loads are directly connected while
the DC loads need the AC/DC power converters in order to be
connected to the LVAC networks. On the other hand, the DG units
which produce DC power output (e.g. solar photovoltaic arrays,
fuel cells and energy storage devices) can be connected to the AC
bus line of the LVAC networks using DC/AC inverters [5,8]. Fig. 1
indicates the typical configuration of the DG units with the AC
power output (e.g. wind turbines) and that with the DC power
output (e.g. PV systems and fuel cells) connected to the LVAC
network.
2.2. Definition of LVDC networks
Operation of today's consumer equipments and tomorrow's
distributed renewable energy generating units needs us to ponder
the alternative energy systems. With the concept of smart grid
coming up, the LVDC networks such as those used for industrial
power supply and commercial buildings are increasing daily [73].
In the future, the DC distribution system will become an alter-
native way to supply all the electrical equipments connected by a
bus system and optimally controlled by an energy management
system (EMS). As an example, the PV systems have a highly
modular structure with many possible configurations available in
which the PV inverter can be connected [5]. The emerged config-
urations are designated as the central inverter or string inverter. In
that way, the DC based DG units and energy storage devices
produce the DC power which would be easily connected to the DC
bus line or LVDC network. An ESS can also be charged/discharged
with the LVDC network and loads (AC and DC loads) be connected
[6,7]. In this case, the AC power generating units need an AC/DC
power converter for their connection to the LVDC network [8–10].
Fig. 2 depicts the LVDC network with DG units connected to it via
the common bus bar at PCC. To this end, the AC based DG units
(wind turbines) connection to the LVDC network requires inverters
while the DC based DG units are directly connected as indicated in
the figure.
2.3. Microgrid systems
In a broader and futuristic manner, microgrids (MGs) are tiny
power systems which embed various components such as con-
trolled and uncontrolled loads, DG units and storage devices
operating together in a coordinated manner with controlled power
electronic devices (active and reactive power flow controllers,
frequency and voltage regulators) which are integrated with
protective devices [1,2]. They can be operated based on the
principles of the AC power systems (i.e. AC microgrids) or DC
power systems (i.e. DC microgrids). Thus, the architecture of the
future energy system will eventually look very different from that
Fig. 1. Typical configuration of the DG units with LVAC network.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405 389
4. of the conventional energy system along with the MGs expected to
be the main building blocks [66,67]. In this context, the future
energy system i.e. “The smart grid” is anticipated to have the
following features [68–70]:
Energy efficiency, sustainability and RESs inputs.
Reliability, security, ESSs and DG units (renewable based).
Sensing, measurements and advanced control methods.
Load usage awareness, real-time EMS and advanced load
components (e.g. electric vehicles, heaters and industrial motor
drives).
Integrated information and communication infrastructures full
of cyber-security.
The above features can satisfactorily be achieved through the
use of the MGs and often at lower cost with greater efficiency than
measures applied to the conventional AC grids. Further of this, the
MGs can benefit both the utility and the customers as follows:
To the utility: The MGs are treated as controllable entities that
operate as a single dispatchable unit (load or generator) to
provide power or ancillary services and meet the needs of the
upstream networks. Moreover, the MGs involve alternative
energy sources which can offer far higher efficiency and reduce
environmental degradation contrary to most conventional
power generation units [71]. They are also strategically
installed near the loads to provide a variety of benefits such
as network voltage and frequency regulation if properly oper-
ated [74].
To the customers: The MGs can provide both thermal and
electricity needs to meet special demands such as local relia-
bility, and can improve power quality by supporting local
voltage and frequency [72]. They not only reduce the effects
of voltage sags but also offer uninterruptible power supply
functions as may be needed in the areas with critical loads,
such as banking systems, semiconductor industries, hospitals
and data centers [73]. It is also possible for microgrids to suit
the needs of an area it would be serving. That is to say, the MGs
can be the fundamental power source that provides energy for
lighting only as required in most of the rural areas [75,76].
Next, the MGs can be operated in a grid-connected mode or an
autonomous islanding (standalone) mode, and these two opera-
tion modes of MGs can be briefly described as follows: When tied
to the utility system (i.e. grid-connected mode), it is known as
“On-grid mode”. In this mode, the MG is usually connected to the
main medium voltage (MV) e.g. 11–66 kV, or low voltage (LV) e.g.
110–690 V networks depending on the location and the total
capacities of the installed DG units [77]. It either receives or
injects some amount of power into the main system and provides
power support to its local loads. Moreover, the main function of
the DG units is to generate power and provide local and power
support in the microgrid systems. With the interfacing power
converters, controllable active and reactive powers can be pro-
duced in MGs whereby their reference values of each DG units are
commanded by the microgrid operating manager (MOM) [78,79].
Also, some DG units can be controlled to track their maximum
power point (MPP) as in wind turbines and PV arrays. Upon fault
occurrence and its subsequent switching incidents, or preplanned
switching events, the MG is disconnected from the utility as soon
as possible and picks up its local loads. In this case, the MG is in
islanding mode also known as “Off-grid mode” and it operates
autonomously in a similar way to the physical islands when
disconnected from the main network. The least important loads
can be shed if the power capacity of the MG is insufficient to
support all its local loads [80,81].
On the other hand, the future energy system is anticipated to
be based on the various DG units, storage devices and controllable
loads all networked with the advanced information and commu-
nication devices. In such an energy system, the MGs, DG units and
ESSs can be aggregated and be used as the “virtual power plant
(VPP)” main building blocks [1,82]. In contrast with the MGs, the
VPPs are the new concepts which consist of aggregated DG units
that are treated as a single entity. Typically, the individual
resources would be small but pooled together to form a size
which can provide reactive power or support peak power demand.
Given their multiple locations with different generation technol-
ogies, it will be easy to see how controlling a group of small,
dispersed DG units would be facilitated by ICT-assisted grid
control. In that case, the power system will rely upon software
systems to remotely and automatically dispatch and optimize
generation, demand-side or storage resource in a single, secure
web-connected system [83]. In fact the VPP will represent an
“internet of energy”, whereby tapping the existing grid networks
to tailor electricity supply and demand services for a customer,
maximizing the values for all utility participants through applica-
tions of the ICT [84]. Fig. 3 illustrates the details about the
concepts of the VPP and MGs.where the abbreviations HAH—
Home Area Network, BAN—Building Area Network, IAN—Industrial
Area Network, FAN—Field Area Network, NAN—Neighborhood
Area Network, AMI—Advanced Metering Infrastructure and CCLs
—Customers with controllable loads.
3. Feasibility of AC and DC microgrid systems
Electric grid delivers electricity from the point of generation to
consumers, and the electricity network functions via two primary
systems, namely: (i) transmission system and (ii) distribution
system. The transmission system delivers power from power
plants to the distribution substations while the distribution
system delivers power from distribution substations to the con-
sumers. In that case, the centralized power stations are usually
located quite far from the load centers. It is therefore difficult for
the power system operators to monitor and address the distur-
bances occurring at the load centers. However, the DG units
connected to the grid at low voltage level in the form of MGs
are gradually changing the structure of the conventional grid from
passive to active distribution networks [11]. Their interconnection
with electric power system in the distribution level opens new
Fig. 2. Typical configuration of the DG units with LVDC network.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405
390
5. possibilities of easily controlling any component in the network.
Furthermore, in the near future, customers are expected to be able
to generate, store, control and manage part of the energy they
consume through the deployment of microgrids [16,70]. The
future energy system is anticipated to incorporate more intelli-
gence devices installed in the generation units, transmission lines,
substations, and distribution networks and some controllable
loads. It will be a combination of both power system and
information and communication system networks. The two net-
works will be embedded to form a more advanced architecture
whereby the flow of power and information will be a two-way
system, allowing customers to have access to the state of the grid
such as: (i) current peak power demand, (ii) the electricity tariffs
at the energy market and (iii) how to minimize their bills. In so
doing, customers can easily communicate and sell their excess or
stored energy to the utility at the reasonable price. Again, the
electricity market participation for any DG unit will be the key
aspect of the microgrids and VPP concepts. Depending on the
combination of the DG units, flexible loads and storage units,
customer's participation in different electricity markets will be
possible [85]. At the moment it is not profitable for a small DG unit
to participate in the market, because most of the countries e.g. in
Europe where these concepts have been widely spread, have feed-
in tariffs which are more attractive. But the tariffs decrease yearly
with a given percentage depending on the DG unit technology.
So, in the future the participation of the microgrids in the form
of the VPP in different energy markets could be essential. One big
advantage of the VPP is that the whole cluster can be managed by
one broker or trader, reducing the market participation costs for a
single unit significantly. In that case, the individual owner of some
DG units can generate power and store the excess generated
power in the ESSs. The stored energy may be discharged during
peak demand to earn income [86]. Therefore, with the AC and DC
microgrid systems equipped with the DG units, ESS and mixed
controllable AC and DC loads, customers are expected to have
information access of every unit connected to the given microgrid
system. This reflects the idea of the future energy system of which
the electrical and communication infrastructures expected to be
embedded together [82]. Therefore, the literature survey of the AC
and DC microgrid systems is provided in the Sections 3.1 and 3.2,
respectively.
3.1. AC microgrid systems
A typical AC microgrid systems interconnected with MV system
at the PCC is shown in Fig. 4. The main system could be an AC or
DC bulk system. The DG units and ESS are connected at some
points within the distribution networks. Part of the network
consisting of the DG units and load circuits can form a small
isolated AC electric power system i.e. an ‘AC microgrid’. During
normal operating conditions, the two networks are interconnected
at the PCC while the loads are supplied from the local sources (e.g.
the RES based DG units) and if necessary from the utility. If the
load demand power is less than the power produced by DG units,
excess power can be exported to the main system. Table 1(a)
indicates some typical examples of the AC powered loads that are
available in our homes. Meanwhile, Table 1(b) shows some typical
examples of AC MG systems installed in various countries or used
as testing prototype. In most cases, the AC microgrid system
operations adopt the voltage and frequency standards applied in
most conventional distribution systems.
In the literature, the AC microgrid systems with renewable
based DG units have been researched and implemented in various
countries. Their operating feasibility is discussed by a number of
researchers. As per Solanki et al. [36], a smart energy management
system (SEMS) to optimize the economic operation of the micro-
grid is presented. The SEMS consists of DG unit forecasting, ESS
management and optimization modules. The characteristic of the
PV output in different weather conditions is studied and then a
1-day-ahead power forecasting module is presented. As energy
storage needs to be optimized across multiple-time steps, con-
sidering the influence of the energy price structures, their eco-
nomics are particularly complex. Therefore, the ESS module is
applied to determine the optimal operation strategies while the
SEMS integrates the smart management of the ESS, economic load
dispatch and operation optimization of the DG units. More
about optimization of the DG units in microgrids can be found in
[67–69]. An overview of the rural areas poverty and lack of access
Fig. 3. Typical configuration of VPP system.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405 391
6. to energy, especially to electrical energy, are still significant in the
rural regions across the global. Nearly 70% of the population in
sub-Sahara African countries lives in rural areas and less than 10%
of this population has an access to reliable electricity [71]. In that
case, the MGs with local renewable energy resources such as solar
PV arrays, wind turbines or biomass are proposed as an alternative
isolated system to rectify the problems, and consequently support
their socioeconomic development.
As per Jiayi et al. [72], a review of distributed energy resources
(DERs) with several technologies and MG technology are pre-
sented. The authors describe the MG operation in both grid-
connected and island mode with the market environment of the
MGs being also presented. Preplanned switching and fault event
leading to islanding of the distribution subsystem and formation of
microgrid is presented in [73]. In that sense, the DG unit interface
converter with independent real and reactive power control helps
to minimize the effects of transient state and maintain both the
angle of stability and voltage quality within the microgrid. Guer-
rero et al. [14] present the hierarchical control based on the droop
control methods in the AC and DC microgrids with multilevel
control schemes. More emphasis in the microgrid control strate-
gies is also given in [39,40,43].
Moreover, Guerrero et al. [97] discuss the parallel operation of
the DG unit inverters by using resistive output impedance without
communication signals. Meanwhile, wind energy is being consid-
ered as the fast penetrating renewable energy source into the
power system. The variable speed wind turbines e.g. doubly-fed
induction generator (DFIG) and permanent magnet synchronous
generator (PMSG) can be used to improve the performance of the
AC MGs during islanding mode of operation. The authors in [59]
report a dynamic model with DFIG to provide an additional
support for primary frequency and voltage regulation within the
MG network. Using DFIG control flexibility, the AC microgrid
transient and dynamic behaviors during islanding and grid-
connected modes are improved. Besides, synchronization of
microgrids with the main system has been a hot research topic
across the world. Ref. [47] presents the control of the MG with
active synchronization. A review on control strategies is given
together with discussion on the challenges encountered when the
ESSs are included within the AC microgrids [48,49]. Furthermore,
the authors in [54–56] report the study of power converter control
and protection schemes for grid-connected microgrid systems
integrated with renewable based DG units.
The impact of different control schemes on system stability during
subsequent and fault-forced islanding conditions for microgrid sys-
tems with inverter based DG units has been analyzed. The authors in
[54] present the protection techniques in the grid-connected and
islanded modes of operation for the MG systems. In this case, the
authors illustrate the effects of the generation to load ratio on critical
clearing time and identify the factors which influence AC MGs stability.
The power control strategy developed for the low-voltage microgrid is
detailed in [64]. This strategy comprises a virtual inductor at the
interfacing inverter output, an accurate reactive power control and
sharing algorithm which consider the impedance voltage drop and the
effect of the DG unit local loads. The virtual inductance can effectively
prevent the coupling between real and reactive powers by presenting
mainly inductive impedance even in a low-voltage network with
resistive line impedances without any physical connection. On the
other hand, the black-start restoration sequences to be used for MG
after a blackout are solved by Moreira et al. [65]. The proposed
solution is based on the communication infrastructures which auto-
matically identify and restore the special issues for the MG services.
Fig. 4. AC microgrid structure with the DG units and mixed types of loads.
Table 1(a)
Typical examples of home AC powered appliances.
Sl. no. AC powered
loads
Voltage
ratings (V)
Current
ratings (A)
Power ratings
1 Microwave oven 240 − 8.3 kW
2 Dishwasher 120 11 −
3 Toaster 120 − 1050 W
4 Coffee pot 120 − 1100 W
5 Electric clothes drier 240 − 5600 W
6 Electric cook-top 240 − 8.8 W
7 Refrigerator 120 8.3 −
8 Washing machine 120 7 −
Table 1(b)
Typical examples of AC microgrid systems.
AC microgrid components Voltage level Frequency
(Hz)
Capacity
PV arrays, diesel generator, battery banks—Kythnos,
Greek Island [101]
1—phase
LVAC
50 12 kWp from PV system, 85 kWh from battery bank
PV arrays, wind turbines, controllable digester gas engines and
lead acid battery banks—Hachinohe, Japan [102]
Not specified 50 Demand¼610 kW; power generated by PV arrays and wind
turbine¼150 kW, digester gas engine¼510 kW, battery¼100 kW.
PV arrays BESS—rural areas in Senegal [66] 220 V 50 Hz 0.5–10 kW per household
Two steam turbines, two diesel generators and
converter-interfaced source [69,73]
94 V (pk),
115 VLL (rms)
60 Hz 3 MW
Gas engines (GEs), wind turbine (WT), EDLC and BESS, Japan [8] 110, 230,
240 V
50, 60 Hz 50 kW–2MW
Fuel cells, PV arrays, WTs, ESSs and AC utility;
Porto, Portugal [77]
400 V 50 Hz 50–200 kW
More other studies are in
[2,8,39,40,47,48,61,64,65,67,105,110,124]
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405
392
7. More about the AC MGs operation and their ongoing researches can be
found in [86–89,91,95–98].
By referring to the literature above, one can conclude that the
AC MGs are feasible with both renewable and non-renewable
energy sources. They are involved in many areas of applications
such as in remote areas, commercial buildings and as backups for
power supply and improve the efficiency and reliability of the
existing power system infrastructure.
3.2. DC microgrid systems
Traditional electric power system was designed to move central
station alternating current (AC) power, via high-voltage AC (HVAC)
transmission lines and lower voltage distribution lines to house-
holds and businesses that use the power in incandescent lights, AC
motors and other AC equipments. Meanwhile, the DC power
systems have been used in industrial power distribution systems,
telecommunication infrastructures and point-to-point transmis-
sions over long distances or via sea cables and for interconnecting
AC grids with different frequencies. Today's consumer equipments
and tomorrow's DG units are dominated by power electronics
devices. These devices (such as computers, fluorescent lights,
variable speed drives, households, businesses, industrial appli-
ances and equipments) need the DC power for their operation.
However, all these DC devices require conversion of the available
AC power into DC for use, and the majority of these conversion
stages typically use inefficient rectifiers. Moreover, the power from
DC based DG units must be converted into AC to tie with the
existing AC electric network, only later to be converted to DC for
many end users. These DC–AC–DC power conversion stages result
in substantial energy losses. Using the positive experiences in the
HVDC operation and the advance in power electronics technology,
interests in pursuit for effective solutions has increased. The LVDC
distribution network is a new concept which is one possibility to
tackle the current power distribution problems and realize the
future power system [11]. It has the features that meet the new
requirements of the electrical distribution networks. Fig. 5 shows
the typical DC MG systems interconnected with the main systems
at PCC which can be medium voltage AC (MVAC) network from the
conventional power plants or an HVDC transmission line connect-
ing an offshore wind farm. Table 2(a) depicts some typical
examples of the available home DC powered appliances while
Table 2(b) shows the typical examples of the DC MG systems
used as testing prototype or typically installed for most of the
data centre or critical load applications. In the case of DC MG
configurations, the low voltage DC links are based on bipolar
configurations where the loads can be connected across the
positive polarity and the ground or between the two polarities.
The LVDC distribution network can improve the efficiency of
delivering power to the distribution network. It ensures a higher
power quality to the customers than in the present distribution
network and facilitates more DG units' connection [12]. Measuring
instruments such as advanced metering infrastructures (AMIs),
demand energy managements (DEMs) and protection systems can
also be integrated into the power converters. To achieve all these,
ongoing researches to find out the details and more emphasis on
the DC MGs applications such as power supply for the critical
loads in commercial buildings, electronics factories and hospitals
have been presented. Results indicate a significant reduction in
power quality problems, losses and downtime and protection
malfunctions [73–75].
The research road map in [4] presents the opportunities and
challenges associated with a DC distribution system for industrial
power system. In that article, the focus is directed on the interac-
tion between the power converters and the challenging issues of
the system grounding. The DC MG for small-scale residential
houses is investigated to find out the influence of current balan-
cing, system losses together with its stability [6,7]. The authors
concluded that; the DC MGs have attractive features in terms of
simple structure, low system cost and the overall improved
efficiency since few power converters are needed compared to
the AC MGs [10–12]. An isolated DC network which comprises
solar photovoltaic (PV) systems and unbalanced AC loads was
investigated. In this study, the author used the LVDC cable to
interlink the PV based DG units and their corresponding local
loads [15,60]. Also, the DC MGs control strategies in grid-
connected and islanding mode are presented by several authors
[15,45,46]. For instance, the control strategy for the grid-
connected DC MG with renewable based DG units and ESSs is
reported by Chunhua Liu et al. [17]. Nevertheless, a detailed
discussion on control strategies for the battery energy storage
system during islanded and grid-connected operation to adjust the
DC bus voltage is studied and presented in [45]. More details about
the control strategies for grid connected and islanded DC MGs
operation are available in [50].
Protection issues in the LVDC distribution system are a challen-
ging problem addressed by few authors. In [51], the authors
propose the philosophy of protecting the DC MG systems with
the DG units, sensitive loads and power converters. The grounding
system, protection devices, power converters and battery protec-
tion methods [53,54], and DC feeder protection approach together
with their coordination are stated as the potential areas in
designing any effective protection scheme in the LVDC distribution
systems. More about the DC MG protection mechanism is given in
[55–57]. Moreover, the dynamic behavior and stabilization of DC
MG with constant power loads are also given in detail by Alexis
Kwasinski and Onwuchekwa [45]. In most of the references above,
the DC MG systems or LVDC networks are employed to supply
power to highly sensitive electronics loads and industrial power
systems in which motor drives are mostly used. In these areas of
applications, the LVDC networks have shown high performance
against power malfunction. The electric vehicles (i.e. PHEV and EV)
charging stations are the other areas where the DC MGs are most
likely to have significance to the future energy system [17,60,124].
In this perspective, with the deployment of a smart grid, the
vehicle-to-grid (V2G) concept could widely become a reality. The
V2G implementation would allow the EVs and PHEVs to also act as
suppliers to the grid [87,88]. For example, production from the
renewable based DG units such as wind power and PV system is
usually intermittent. For their optimal utilization, the ESSs could
be needed but often not available or limited. If the wind power
production is too high during low time intervals, surplus power is
Fig. 5. Concept of a DC microgrid system with the DG units and mixed types
of loads.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405 393
8. often wasted and system security might not be compromised.
However, with the V2G technology in smart grid, the surplus
energy can simply be stored and later be supplied back to the grid
as required.
To be more specific, a recent study on optimal planning and
operation of the smart grids with the EV interconnection has
shown that the EV-ESSs can be used to reduce utility-related
energy costs. Also, by offering different kinds of ancillary services,
it can control and manage part of the smart grid or commercial
buildings following the arbitrage of energy between buildings
with different tariffs [89]. Referring to more literatures regarding
the EVs operation in the low voltage distribution networks,
authors in [90] reported the influence of the EVs charging stations
to the Polish power network and showed how the grid behaves
when overloaded during peak demand. Such ongoing research
studies in the DC MGs with the DG units powered by renewable
and non-renewable energy sources significantly prove their feasi-
bility. Therefore, power from substation or DG units and ESSs can
be transmitted using a DC distribution line with single cable
(monopolar dc link), two cables (bipolar dc link) or even three
cables (homopolar dc link). The first two arrangements are
normally used for HVDC long distance transmission system while
the last is mainly used in low voltage distribution system (LVDC
network). In this subsection, the three types of DC link configura-
tions are briefly discussed.
3.2.1. Monopolar DC link
This type of DC link configuration uses one high voltage
conductor and a ground-return or sea-return as indicated in
Fig. 6(a). It is advantageous from economic point of view, but is
prohibited in some countries because the ground current causes
the corrosion of pipe lines and other buried metal objects.
However, this type of configuration is already in operation in
some European countries (e.g. Italy–Greece HVDC link) and most
of them are used for submarine crossings. A metallic return can
also be used where concerns for harmonic interference and/or
corrosion exist. Since the corona effects in a DC line are substan-
tially reduced with negative polarity of the conductor as compared
to the positive polarity, a monopolar link is normally operated
with negative polarity.
3.2.2. Bipolar DC link
Fig. 6(b) shows the bipolar DC-link configuration with two
conductors, one operating at positive-polarity and the other
operating at negative-polarity. The junction between the two
power converters may be grounded at one or both ends. However,
if both ends are grounded, each link can be independently
operated when necessary. Each terminal has two sets of the power
converters of equal rating in series on the DC side. Since both poles
operate with equal currents under normal operation, there is zero
ground current flowing under these conditions. Monopolar opera-
tion can also be used in the early stages of the development of the
bipolar link. However, under faulty converter conditions the
system can transmit half of the power in monopolar mode. In this
case, one DC line may be temporarily used as metallic return with
the use of suitable control strategies. Besides, the monopolar mode
of operation can be maintained for a limited time only. In addition,
the loads and DG units can be connected in parallel across these
two lines [11,12].
3.2.3. Homopolar DC link
The homopolar link, whose configuration is shown in Fig. 6(c),
has two or more conductors having the same polarity. Usually
negative polarities are preferred and can be operated with ground
return or metallic return. Due to undesirability of a DC link with
ground return, bipolar links are mostly used. The homopolar link
Table 2(b)
Typical examples of DC microgrid systems.
DC microgrid components Voltage range Capacity
Sweden UPN AB [103] for Data center IBM 24–350/380 LVDC (bipolar DC-link) ≥5 MW
Japan NTT Group [103] for data centers 380/400 LVDC (bipolar DC-link) ≥5 MW
New Zealand Telecom NZ [103] for data centers 220 LVDC (bipolar DC-link) 0.5–5 MW
US Intel Corp. [103] for data centers 400 LVDC (bipolar DC-link) ≥5 MW
For general case [75] 187.8 V—450 LVDC 600–2100 W
Two Steam turbines-Testing prototype [77] 800 V—1200 LVDC 4.8–18 kW
PV arrays, BESS AC utility system [9] 180–210 V LVDC (system model) 150–945 W
360 V—420 LVDC (for experiment prototype)
PMSG WTs, BESS AC utility system [10] 1200 LVDC 0.9–3.5 MW
Gas engine cogeneration, EDLC, BESS, PV arrays AC system [12] 7170 V, 340 LVDC (bipolar DC-link) 700–2700 W
For general case [45] testing prototype 200, 400, 415 LVDC 5 kW, 15 kW
More studies are also reported in [11,14,16,47,48,51,60,73]
Table 2(a)
Typical examples of home DC powered appliances.
Sl. no. DC powered loads Voltage ratings (V) Current ratings Power ratings (W)
1 Laptop computer 20 4.5 A −
2 Cell phone 5 550 mA −
3 Wireless phones 6.5 500 mA −
4 DVD player, home theater system − 26, 300
5 Battery powered vacuum cleaner 10 250 mA −
6 Cable modems 12 750 mA −
7 Wireless internet router 5 2.5 A −
8 Powered USB port 5 3.8 A −
9 54″ plasma TV − − 465
10 PC mini-tower − 6 A
11 Variable speed drives for washers, dryer or air-condition 380 − −
12 Rapid charger—PHEV or EVs 200–380 − −
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405
394
9. has the advantage of reduced insulation costs. However, the
disadvantage of the earth return outweighs the advantages. The
three-wire system has the highest efficiency of all the three links
and is mainly used for DC distribution system from substation to
the actual consumers. It consists of two outer wires and a neutral
wire. It has the advantage that the voltage is divided between the
two sets formed by these three wires. For example: If the voltage
between one set is 220 [V], this can be used for domestic purposes
by using the neutral and one outer wire; while the voltage level of
440 [V] can be obtained by taking the voltage between the two
outer wires utilized for industrial purposes [16].
Therefore, due to the importance of using the DC MG systems
in the future energy systems, the paper presents their comparison
with the AC MG systems. Operation and control of both microgrid
systems and their protection approaches are presented in the next
section.
4. Operation and control of microgrid systems
One of the elements of the future energy systems is the dis-
tribution network that measures and controls the usage [36,70].
In such energy systems, the power generation depends on the
market situation (supply/demand and cost) and the power source
available [66]. These structures are more realizable in the form of
AC and DC MG systems. They are resurging due to the develop-
ment and deployment of the RESs, advance in power electronics
and their inherent advantages in various areas of applications [4].
The power electronics interfaces (PEIs) play the most important
roles in interfacing various components within the MG systems
and all technical issues such as power balance and power quality
are resolved via the control of these devices [5–7]. The PEIs allow
the MG systems to operate in both grid-connected mode and
islanded mode while providing a high quality of power with
minimum equipment cost [91,118]. In that manner, the PEIs are
required/expected: (i) to provide the fixed power and local voltage
regulation, (ii) to facilitate the DG unit to fast track the load
demand using the energy storage devices, (iii) to incorporate
the control methods for load sharing between the DG units, and
(iv) to integrate the various key technologies for successful
modern or future smart distribution systems [92–94]. So far, the
DC MGs do not have demerits of the AC microgrids such as
synchronization of frequency and difficulties in voltage control
during islanding operation [95–97]. That is to say, the DC MGs
are much simpler in operation because the frequency or phase
control structure is unnecessary as compared to their counterpart.
Therefore, presented in the next subsections are the components
configurations, control and power managements in both AC and
DC MG systems.
4.1. Microgrid structural configurations
AC and DC microgrid systems are the central interest of this
paper, as their structural configurations attract the easy deploy-
ment of the renewable based DG units and ESSs to achieve several
benefits as mentioned in the previous sections. Fig. 7 shows the
interaction between the two types of electrical systems and their
corresponding components interlinked by the power converters.
From this figure, it can be revealed that the AC MGs (or LVAC
networks) can be connected to the bulk AC system via power
transformers and require an AC/DC power converter to intercon-
nect with the bulk DC system. That means, the AC based DG units
together with AC loads can directly be connected to the LVAC
networks. On the other hand, the DC based DG units and the DC
loads require DC/AC and AC/DC power converters for their con-
nection into the LVAC network, respectively. In the case of the DC
MGs (LVDC networks), bi-directional DC/DC (boost) and DC/AC
(inverter) power converters to interconnect with the bulk DC and
AC systems are needed, respectively.
Furthermore, the AC based DG units and AC loads can be
coupled with the DC MGs using the AC/DC power converters to
facilitate their stable operation. On top of that, the DC MGs can
eliminate the DC/AC and AC/DC power conversion stages required
by the AC MGs to connect the DC based DG units and DC loads,
Fig. 7. Interconnection of various microgrid components.
Fig. 6. Concept of the DC link configurations based on: (a) monopolar type, (b) bipolar type and (c) homopolar type.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405 395
10. respectively. In addition to the above, the power converters do not
have a transformer, which contributes to overall system down-
sizing. This means, the DC MGs are more advantageous in terms of
system efficiency, cost and system size than their counterpart.
4.2. Microgrid control and power management
The use of the power converters to interconnect various compo-
nents in MGs requires proper control strategies [68]. The controllers
must be able: (i) to import/export energy from/to the bulk network,
(ii) to control the real and reactive power flows and manage the DG
units connected to it and (iii) to operate within its prescribed
specifications i.e. the system frequency and voltage must be main-
tained within their predefined limits. Usually the voltage and
frequency variations are very small if the MG is connected to strong
grids. However, large variations may occur in autonomous grids
[33,46,49]. Moreover, in the grid-connected MG systems, inverters
use the signal from the main grid as a reference to obtain the signal
with the correct frequency (in case of AC MGs) and voltage (in case of
AC and DC MGs) needed for the power factor correction and current
reference computation [39,43,115]. Each DG unit and energy storage
device operating in this mode can therefore be viewed as a current
source, whose power flow can be controlled by varying the current
reference. In both AC and DC MG systems, the grid maintains the
system stability [12]. However, in islanding mode the reference from
the main grid is lost and a new reference voltage must be internally
generated by the individual DG unit controller to continue generation
of good power quality. In this case, the DG units connected to MGs
thus appear as controlled-voltage sources whose outputs should
rightfully share the load demand in proportion to their ratings [119],
so as not to overstress any individual unit [15,40,41]. On that note,
power management strategies are vital for the MG systems operating
autonomously in the presence of few small DG units, where no single
dominant energy source is present to supply the power demand [42–
44].
Many researchers have addressed the operational and control
issues in MG systems to allow their stable operation in both grid-
connected and islanding modes [78–80]. With most of the DG
units widely dispersed, it is impractical to interlink every compo-
nent in the microgrid systems by wires. Nevertheless, the future is
in our hands with advanced information and communication
technologies (ICTs) and it is clearly possible with the emerging
smart grid technology to interlink every component in microgrids
[43]. Currently, to avoid the unnecessary wiring, measurements
are taken locally within the vicinity of each DG unit and the droop
control methods are commonly used with many improved var-
iants to allow proper sharing of real and reactive power within the
MG system [77–79]. Their main functionality is to introduce some
virtual inertia to the DG units. In so doing, each DG unit behaves
like a synchronous generator connected to the traditional power
system, whose power balancing ability can be emulated.
In opposite to the droop control methods, the master–slave MG
control method is reported [80]. In this concept, the master–slave
configuration avoids currents circulation among the power con-
verters of the DG units and variation in frequency and voltage of
the MG system. Unfortunately, this approach decreases the relia-
bility of the system by placing an extra burden on the criticality of
the master source and the interconnections between slave inver-
ters and the power distribution unit. It also decreases the stability
of the system. On the other hand, it is an easily-using strategy to
avoid circulating currents and special steps can be applied to avoid
the aforementioned limitations [47,52].
Most of the recent research interests have largely focused on
the droop control strategies to allow the MGs to operate inter-
changeably between the two modes of operation. For instance, the
ISA-95 [15] is adopted in the MG systems control with the zero to
three control levels usually implemented in a structural form
depicted by Fig. 8(a). The challenge with the MG controllers
remain to be the need for operating each individual DG unit at
its optimal state and to coordinate different advanced technologies
with mixed loads and ESSs [97,98]. In addition, the islanding
detection algorithm is another challenge which is necessary to
guarantee the MGs to operate interchangeably while attaining a
seamless transition between the two modes [99,100,106].
Traditionally, the MG controllers ensure the system security,
optimal operation, emission reduction and smooth transfer from
one operating mode to the other without violating system con-
straints and regulatory requirements [63,116]. These objectives can
be achieved through three kinds of controllers, namely: (i) a DG
unit controller (DGC) connected to each unit and energy storage
device, (ii) a microgrid central controller (MGCC), and (iii) a Load
controller (LC). Each DG unit can produce its output frequency and
voltage amplitude tuned in proportion to the amount of active and
reactive power drawn so as to achieve a proper load demand
sharing [64,96,97]. Based on these requirements, the control in
microgrid systems is therefore divided into four levels which are:
Level 0 (inner control loop): This is a power controller loop that
determines the operating state of the DG units and storage
devices and it is known as low level voltage and current
controllers [40,44,48]. Fig. 8(b) illustrates the configuration of
this control level in which the regulation issues such as the
driving signal of each module are integrated. It also includes
the feedback and feed-forward compensators together with the
linear and non-linear control loops. Maintaining the system
Fig. 8. Detail configurations of microgrid system control levels: (a) hierarchical
control levels in microgrid systems; (b) configuration of the secondary, local and
inner control loops in microgrid systems; and (c) configurations of the global and
secondary control structures.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405
396
11. stability with high bandwidth and performance, a fast response
under any operating condition is usually guaranteed in this
level.
Level 1 (local control loop): It is also known as the primary
control or decentralized control (they do not require any
commutations between the power converters) [32,46]. The
droop control equations are often used in this level to emulate
physical behaviors that make the system stable and more
damped [8,12,14]. For example, it includes the active power/
voltage droop for the DC microgrids [13], and active power/
frequency and reactive power/voltage droop characteristics
[77,78] for the AC MG systems.
Level 2 (secondary control loop): It is located in microgrid central
controller (MGCC) and responsible for measuring frequency
and voltage [44,55,64]. The output of this control unit is sent
through communications to adjust the reference of the local
controllers (e.g. P/Q droops) as indicated in Fig. 8(b). In
addition, it can include a synchronization control loop to
seamlessly connect or disconnect the MG to or from the MVAC
or MVDC networks [46].
Level 3 (global control loop): It is also called a tertiary control
loop in which the energy-production or energy-market stage
which controls the power flows between the MGs and the grid
is implemented [14,49]. In this level, once the MG is connected
to the main system the power flow can be controlled by
changing the voltage inside the MG. Fig. 8(c) illustrates that
the frequency and voltage references for the secondary control
are generated through a MG operating manager (MOM) or the
transmission system operator (TSO). It also organizes the
relation between a given MG and distribution network as well
as other connected MGs and forecasts the short-term load
changes, storage capabilities together with marginal generation
cost of each DG unit [97]. The global or emergency control,
and to some extent the secondary control are termed as the
centralized control systems, since they both require commu-
nication for their operation [33,48].
Fig. 9 indicates a typical configuration of the MG system with
the centralized control based on MGCC, DG units and controllable
loads. The DG units and energy storage devices are fitted with the
DGCs that execute smooth and flexible operation to meet custo-
mer and utility requirements. The DGCs may operate with or
without any intervention of the MGCC [48,64]. The objective of the
DGCs is to take care of the local control functions (levels 0 and 1)
which greatly depend on power electronic interfaces provided at
each DG unit and storage device.
In summary, the DGCs ensure: (i) new microsources can be
added to the system without modification in the existing AC or DC
MG configuration, (ii) MG can connect/disconnect itself to/from
the utility in a rapid and detectless style, (iii) active and reactive
power can be independently controlled, (iv) voltage sag and
system unbalances can be corrected, (v) faults can be handled
Table 3
Comparison between AC microgrids and DC microgrids control strategies.
Operating mode AC microgrids DC microgrids
Grid-connected mode
MGCC
Monitoring system diagnosis by collecting information from the LVAC
network, DG units and loads (AC DC).
Performing state estimation and security assessments, evaluate
economic generation scheduling, active and reactive power control of
the DGs units and demand side management functions using the
available information.
Ensuring synchronized operation with the main grid, maintaining the
power exchange at prior contract points.
The main function of the MGCC is to independently control the power
flow and load-end voltage profile of the DG units in response to any
disturbance and load changes.
Participating in economic generation scheduling, Load tracking or
management and Demand side management (DSM) by controlling the
storage devices.
DGCs
Ensuring that each DG unit rapidly picks up its generation to supply its
share of the load in stand-alone mode and comes back to the grid-
connected mode automatically with the help of the MGCC.
Ensuring that each DG unit quickly picks up its generation to supply its
share of the load in stand-alone mode and comes back to the grid-
connected mode automatically with the help of MGCC.
Islanding mode
MGCC
Performing active and reactive power control of the DGs in order to
maintain stable voltage and frequency at the load ends.
Managing load interruption/shedding strategies using demand side
management (DSM) with ESS support for maintaining power balance
and voltage.
Initializing local black start to ensure reliability and continuity of the
service.
Switching the microgrid to grid-connected mode after the main grid
supply is restored without hampering the stability of either grid.
Independently control the power flow and load-end voltage profile of
the DG units in response to any disturbance and load changes.
Ensuring the DG units rapidly picks up its generation to supply its local
load in islanding mode and automatically reconnect to grid with the
help of the MGCC.
DGCs
Commanding each DG unit to rapidly pick up its generation to supply
its corresponding local loads in the stand-alone mode and
automatically resynchronize to grid with the help of the MGCC.
Ensuring that each DG unit rapidly picks up its generation to supply its
share of the loads in stand-alone mode and comes back to the grid-
connected mode automatically with the help of the MGCC.
Fig. 9. Centralized control of microgrid system with MGCC, DGCs and LCs.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405 397
12. without the loss of stability and (vi) MGs can meet the require-
ments of the load dynamics of the power utility. The built-in
control features of the DGCs comprise: (a) active and reactive
power control, (b) voltage and current control, (c) storage require-
ment for fast load tracking and (d) load sharing through P–f
control. The function of the MGCC is to execute the overall control
of microgrid operation and protection through the DGCs and load
controllers (LCs).
More importantly, the MGCC maintains the power quality and
reliability via active power-frequency, reactive power-voltage
control loops in the AC MGs and active power-voltage control
loop in the DC MGs. It also executes economic generation
scheduling of the DG units, energy storage devices and helps to
maintain the power exchange between the main utility grid and
MG system at mutually agreed contract points. The MGCC also
provides the power dispatch and voltage set points for all DGCs to
meet the needs of the customers. Moreover, the MGCC ensures
energy optimization for the microgrid and maintains the specified
frequency and voltage profile for the electrical loads. This con-
troller is usually designed to operate in automatic mode with
provision for manual intervention when required. Table 3 presents
the comparison between the AC and DC MG systems control
strategies during grid-connected mode and islanded mode with
centralized control system.
Presented in Sections 4.2.1 and 4.2.2 is the detail of commonly
used AC MGs and DC MGs control strategies based on droop
control methods.
4.2.1. AC microgrids control strategy
AC microgrids are now in the cutting edge of the state of art
whereby their control and energy management still require more
investigation. In the grid-connected mode, the AC microgrid
power-frequency (P–f) droop control has been adopted for the
DG units power sharing methods [96–98]. This control approach
uses the grid frequency as a common signal among the DG units to
dynamically balance the active power generation of the system
[101,114,117]. The relationship between the frequency and the
active power output of the given two DG units (DG1 and DG2) can
be expressed by P–f droop characteristics (1) and illustrated by
Fig. 10(a).
f mi ¼ f
n
−KpiPi ð1Þ
where fmi, and fn
are the measured and rated frequency, Kpi and P
are the droop-coefficient and the DG unit output active power
difference between the setpoint and the measured actual active
powers, respectively while i¼1,2,…,λ, with λ being the total
number of DG units.
The differences in the droop coefficients allow the sharing of
the total load power requirement among the DG units according to
a predefined ratio. For example, the total load power requirement
of the MG can be shared in proportion to the rated real power
output of each DG unit [112,120]. The voltage and frequency
regulations are necessary for the local reliability and stability
[46]. Without local voltage control, the systems with high pene-
trations of the DG units could experience over-voltage and/or
reactive power oscillations [48,49,108]. Small errors in the voltage
set points cause circulating currents which can exceed the ratings
of the DG-unit [65,75]. This situation requires a voltage-reactive
power droop controller so that as the reactive power generated by
the DG units becomes more capacitive or inductive the power
converter reference voltage amplitude, Vn
from the measured
reactive power follows its Q–V droop characteristics depicted in
Fig. 10(b) and represented by (2). This has also been reported by
the authors in [95,97,104].
Vmi ¼ Vn
−KviQi ð2Þ
with Vmi and Vn
being the measured and nominal voltage, Kvi and
Qi are the droop-coefficient and reactive power difference
between the setpoint and its measured values, respectively while
i¼1, 2,…,λ, with λ being the total number of DG units.
In the islanding mode, the problems from slight errors in
frequency generation at each coupling inverter and the need to
change power-operating points to match the load demand lead to
restructuring of the control strategies [61]. The active power-
frequency droop characteristics in each DG unit can effectively
solve such problems without any communication network. To
keep the voltage and frequency variation within the defined range,
the gradients/droop-coefficients in (1) and (2) above automatically
adjust to be less gradual [63,110]. However, this process compro-
mises the load sharing accuracy between the DG units [98].
Alternatively, the droop characteristics in (1) and (2) which are
Fig. 10. Control methods for AC microgrid system: (a) P–f droop characteristics and (b) Q–V droop characteristics.
Fig. 11. Droop-characteristics for the DC microgrid system control.
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398
13. indicated by Fig. 10(a) and (b) can further be enhanced by adding
attractive features such as improved dynamics, additional damp-
ing and line impedance effect minimization, as per [75,111].
4.2.2. DC microgrid control strategy
There are several problems associated with the AC MGs, such as
the need for synchronization of the distributed generations, the
inrush currents due to transformers, reactive power flow, harmo-
nic currents and three-phase unbalances [74–76]. Also, the
increase in interest to integrate the DC based DG units such as
solar PV arrays, fuel cells and energy storage systems (e.g.
batteries, super-capacitor modules or hydrolysers) makes a possi-
bility for the DC microgrids [14,42]. The control strategy in the DC
MGs is usually for (i) soft-start control approach, (ii) external
common controller to restore the voltage deviation inside the DC
system and (iii) regulation of the current/power flow from/to an
external stiff DC source which can be a medium voltage DC
(MVDC) system or a DC/AC power converter connected to the AC
grid [97,107,110]. Algorithms for the DC MGs are also necessary to
ensure a smooth transition between the grid-connected and
islanding modes [44] meanwhile, the power sharing between
the DG units in islanded DC MG system can vary from centralized
to decentralized control system [123,124,117].
Unlike the AC MG systems, the DC MG systems are much
simpler in the sense that they do not have reactive power, and
only the voltage quantity available for control is its amplitude. In
this case, the active power is no longer dependent on the system
frequency but the voltage difference [97]. The active power
sharing among the DG units within the DC microgrid can be
obtained through the droop-characteristics (3) as illustrated
below.
VDCi ¼ Vn
DC−βviPi ð3Þ
where βvi is the droop-coefficient introduced to represent the
drooping gradient drawn in Fig. 11, while P is the difference
in active power between the setpoint active power and its
Table 4
Comparison between AC distribution lines and DC distribution lines.
Influence
parameters
AC distribution line DC distribution line
Power transmitted Less efficiency due to high line loss, hence less power
transmission
More efficiency and more power transmission
Require more conductors Require few conductors
System stability Less stable due to easily affected by external disturbances More stable and can also increase the stability of the AC microgrid systems
Reluctance Have reactance in the line No reactance in the line and hence more power transmitted
Frequency
(50 Hz or 60 Hz)
Frequency monitoring is mandatory Frequency is zero, so no need of
Transient stability due line clearance and frequency monitoring
Switching are problems No transient stability problems
Electromagnetic interference must be taken into consideration No electromagnetic interference
Resistance High line resistance and hence high losses Have low line resistance and hence low line losses
Susceptance Charging current and over-voltage problem lead to high cost and
low power transmission
Do not exist, and hence effect of overvoltage and over-charging leading low cost
and high power transmission
Analysis Involve complex numbers and hence difficult to analyze Involve only real numbers i.e. more simple
Table 5
Analysis between AC and DC circuits.
AC circuit analysis DC circuit analysis
(i) (i)
(ii) PLDC ¼ PLAC ¼ PL (ii) PLDC ¼ PLAC ¼ PL
(iii) VLDC ¼ VLAC ¼ VL (iii) VLDC ¼ VLAC ¼ VL
(iv) PLAC ¼ VLIAC cosφ
-IAC ¼
P2
VLAC cosφ
(iv) PL;DC ¼ VLIDC
-IDC ¼
P2
VLDC
(v) VL;DC ¼ VI;AC−VL
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P2
V2
R þ
Q2
V2
X
2
þ
P2
V2
X−
Q2
V2
R
2
s (v) VLDC ¼ VI;AC−VL ¼ IDC R
¼
PL
VL
R
(vi)
PLAC ¼ I2
AC R ¼ P2
V2
2
R
cos2 φ
(vi) PLDC ¼ PIDC −PL
¼ VIDC IDC −VLIDC
¼ I2
DC R ¼
P2
V2
2
R
NB: cos φ¼power factor.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405 399
14. instantaneous shared powers, VDCi and Vn
DC are the measured DC
voltage and its reference value, respectively while i¼1,2,…,λ; with
λ being the number of DG units.
This coefficient βvi must be tuned appropriately according to
the given DG unit ratings and operating conditions since a steeper
slope would give a better power sharing. However, wider voltage
variations may lead to unacceptable predefined limits [31]. Alter-
natively, the improved droop characteristics have been researched
and presented in the various publications to produce a better
response [109,120].
4.3. LVAC distribution versus LVDC distribution lines
Low voltage distribution lines are characterized by a number of
parameters such as power transmission capability, system stabi-
lity, reluctance, line resistance, frequency and susceptances.
Table 4 shows the comparison between the two LV distribution
lines. Operation of the AC and DC microgrids can also be expressed
in terms of the voltage, flow of current and power transport in
their distribution lines and circuits as indicated in Table 5. To
demonstrate the influence of the line parameters, the AC and DC
circuits are considered with only the type of source that differ-
entiates the two circuits. The load has a power factor cos φ with
the same voltage magnitude applied to both AC and DC loads.
where RC and LC represent the conductor resistance and induc-
tance, VIDC and VIAC are the DC and AC source voltages, VL and
PL¼P2 are the load terminal voltage and active power consumed
by the load while Q2 stand for the reactive power consumed by the
AC load. VLDC and PLDC signify the voltage drop and power loss in
the DC circuit, and VLAC and PLAC are the voltage drop and power
losses in the AC circuit, respectively.
5. Microgrid protection systems
One of the major challenges in MG operations is the protection
system to properly respond to both modes of operations [91].
Protection relays must be designed to operate in a grid-connected
mode as well as in an islanded mode. From the literatures, the
fault currents in the grid-connected and islanded MG systems are
significantly different [121,122]. In that case, the protection of the
MG systems cannot be achieved with same approaches that have
been used in the conventional distribution systems. In the grid-
connected mode, the protection relays isolate the MG from the
main grid as soon as possible to protect the DG units and loads. In
the islanding mode, the protection relays operate to isolate the
smallest part of the MG system during fault clearing. Likewise, the
fault currents are limited by the ratings of the power electronics
converters (PECs) to around 2–3 p.u. of their rated currents [50,51].
As stated in the previous sections, the modern MG systems consist
of different distributed generation (DG) units connected to it or to
the utility grid. So a proper coordination between the DG units,
protective equipments and loads is required to ensure a safe
operation of MGs. The protection equipments setting should
always be updated while considering the mode of operation
[53–55]. The MGCC communicates with all the relays and DGCs
to record their status as ON/OFF, their rated current and fault
current contribution. Communication with the relays is required to
update the operating current, detect the direction of the fault
currents and thus properly mitigate the fault [57]. The details of
the generalized relay tripping current that can be used in design-
ing the MG protection scheme are given in Section 5.1, and the AC
and DC MG protection techniques are explained in Sections
5.2 and 5.3, respectively.
5.1. General protection technique in microgrid systems
In designing a protection scheme, safety and fault analysis is
very important. Proper safety model provides appropriate level of
confidence in protection system [54]. In both AC and DC MG
systems safety design, some parameters need to be considered. For
example, sensitivity—in which the nominal threshold values are
set considering all the safety levels of the equipments within the
MG [53]. In many cases, the protection scheme must therefore be
able to identify any abnormal condition. It is worth noting that, the
protection scheme has a role to determine and identify a fault
zone. On that note, selectivity is another important parameter
used to serve this purpose in which a fault is detected in a system
based on voltage magnitude, current and power direction. In order
to minimize fault consequences, the protection system acts to
disconnect only the faulted part. To avoid damage to equipments
and maintain system stability, the protective relays should
respond in the least possible time in any abnormal conditions i.
e. ‘the speed of response’ is another important parameter. Security
level is another factor which must be high so that the protection
system should operate only when required to operate and reject
all the abnormal conditions and transients which are not faults
[55–57].
Basically, there are two main issues that must be carefully
investigated in order to derive a generalized MG protective
scheme. The first is related to the number of installed DG units
within the MG system. Second, the availability of a sufficient level
of short-circuit current in an islanded operating mode of the MG is
very important since this level may substantially drop down after
the isolation from the main grid [55]. Fig. 12 demonstrates the
fault current contributed by each component in a grid-connected
microgrid system. When the MG is operating as a self-contained
power island, any fault currents will have to be supplied by those
DG units which are still connected to it. In this case, their fault
Fig. 12. Fault current contribution in microgrid system with DG units.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405
400
15. current contributions are relatively low values. The difficulty
caused by using the generator units that rely on the power
converters is that these units are designed to limit their output
current to protect their power converter devices [57].
Considering Fig. 12, let the fault current contributed by the bulk
source be IUS, and λ be the total number of the DG units installed
within the given microgrid system, and then the total fault current
contributed by the DG units is given by IDG¼IDG1+IDG2+⋯+IDGλ.
Let also the current rating across the sectionalizers circuit breaker
(SCB) be IR, and then the total fault current at the fault point
without the DG units is given by:
If ¼ IUS ¼ IR ð4Þ
When the DG units are included in the distribution system
(microgrid system), the total fault current at the fault point for the
grid-connected microgrid system is given as
If ¼ IUS ¼ IR þ ∑
λ
i ¼ 1
IðDGÞi ð5Þ
For the MG operating in the stand-alone mode (i.e. off-grid
mode), the total fault current at fault point is given by
If ¼ ∑
λ
i ¼ 1
IðDGÞi ð6Þ
where If is the total fault current at the fault point. So far, the
communication between the central control unit and the protec-
tion relays is necessary to update their operating currents and to
detect the direction of fault currents so as to properly isolate the
faulted region. To generalize this analysis: The relay tripping
current depends on: (i) the operating mode of the microgrid and
(ii) the status of the DG units. Based on (4)–(6) above, the
generalized equation for the relay tripping current is given by
(7) as in [50,55,113].
Irelay ¼ ðIUS μÞ þ ∑
λ
i ¼ 1
ðγi I DG
ð Þ iχiÞ ð7Þ
where μ is the operating mode of the microgrid which takes a
value of one for grid-connected mode and zero for an islanding
mode, γi is the impact factor depending on the distance between
the fault location and the DG unit, and χi is the status of the DG
unit introduced to account for the fault current contribution e.g.
solar PV arrays may have a value of one during the daytime, and a
value of zero at night [56].
5.2. AC microgrid system protection strategy
Several methods of protecting the AC MGs have been proposed
with a wide variety of equipments that are used in the conventional
AC distribution networks protection [55]. These particular types of
protection devices depend on the system element being protected
and voltage level even though there are no specific standards for
the overall protection. The devices which are mostly used for the AC
distribution network protection are: (i) overcurrent relays, (ii)
reclosers, (iii) sectionalizers (SCB), (iv) miniature circuit breakers
(MCB) and (v) fuses. So far, the low voltage distribution protection
schemes or coordination philosophies vary from utility to utility
such as (i) fuse saving schemes, (ii) fuse blowing schemes, (iii)
instantaneous reclose and (iv) delayed reclose.
One scheme which is applied in the AC MGs protection is to
configure each DG unit to have its own relay and operate in the
decentralized mode. This approach is more efficient for the single
line-to-ground and line-to-line faults. However, the method is
limited to faults with low impedance. Another approach is to use a
voltage protection scheme which is a centralized method. In this
case, the phase voltages are transformed into the d-q-0 axes and
then compared with a reference voltage via the MGCC equipped
with central protection unit [50]. When a change in voltage occurs
beyond acceptable range, the tripping device (e.g. miniature circuit
breakers, MCBs or sectionalizers, SCBs) is activated to appropri-
ately isolate the faulted section by sending the signal to the
appropriate relay or switching device as indicated in Fig. 13. The
protection issues and the essential requirements in the future-
protection concept for LV-microgrid or the smart grid can be found
in [52,56]. Based on these papers, the details for microgrid zones
such as MG PCC protection zone with CBs and static switch
transfer (SST), feeder protection with CBs and SCBs, service
connection for customers with MCB and the DG units with MCBs
are reported. Since the realization of the future smart grids with
different modes of operation requires all the technical issues
including protection mechanisms to be resolved, applications of
high speed protection devices such as the standards IEC 61850-
based communication for fast response, selective and reliable
operating scheme, are mandatory [50].
5.3. DC microgrid system protection strategy
In the case of the DC MG system protection, the limits of the
standard SFS 6000 about electrical safety must be fulfilled.
Currently, the main challenges in the DC MGs protection design
Fig. 13. Centralized protection system in the AC microgrid system.
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405 401
16. concentrate to the customer-end inverter, customer network faults
including inverter switching transients and double-fault situations
between different networks (AC and DC networks). Fuses and
automatic relays are also difficult to use with power converters.
This is due to the fact that the power converters do not produce
short-circuit currents as long time as fuses are needed to react
[51,54]. Therefore, a different protection design approach is used
with different fault-detection and grounding methods. In that
case, the protection systems in the DC MG systems consist of the
current interrupting devices, protective relays, measuring equip-
ments and grounding systems [55–57].
Furthermore, there are five protection categories which are used
namely: (i) utility protection, (ii) converter protection, (iii) DG unit
protection, (iv) feeder protection and (v) busbar protection. Example of
the protection devices which are commercially available for the LVDC
networks are fuses, molded-case circuit breakers (MCCBs), power
circuit breakers (CBs) or fast static switches (SSs) and isolated-case
CBs [52,57]. These categories can be operated with decentralized/
centralized system in a similar way as in the case of the AC microgrid
protections. However, the inverters short-circuit current capability
needs to fulfill the ‘used circuit breakers’ or other protection require-
ments. That is to say, the load power converters cannot be dimen-
sioned according to effective power but the short-circuit current
determines their dimension. In a short-circuit situation the inverter
is able to operate up to 2–3 times its nominal current, but a traditional
fuse protection of a low voltage AC network requires fairly high fault
current to operate correctly [54]. For instance, the protection devices
that are commercially available and commonly used in the AC MG
system protection can be directly used to protect against the faults AC
side of the bi-directional AC/DC power converter. These same devices
can be used to protect the AC/DC power converter against faults on its
DC side with different rating considered. If a molded case circuit
breaker which uses a magnetic energizing device trips on instanta-
neous value of the current in the AC side (i.e. Irms), then for the DC
current would be 1.4142 times that of the AC current (i.e.
ffiffiffi
2
p
Irms).
Also, a 50 A fuse requires at least a 250 A short-circuit current to
operate in 5 s (according to SFS ̶ 6000). For this reason, the load
power converter has to be dimensioned larger than the power supply
capacity would require. Fuses must be replaced by much faster
protection relays if smaller power converters would be used. Again,
the AC/DC power converters switching-faults can be protected with
protective functions integrated in it and short-circuit relaying.
Uncontrolled diode bridges cannot control the current so the
components are usually oversized. The active rectifier behaves
similar to the diode bridge during short-circuit so the diodes are
also oversized in this case. If there are some faults in the components
of the power converters (say the controlled rectifiers), these are
detected by over-current relay or by measuring the DC voltage level.
The DC MG faults are covered with combined over-current and
short-circuit protection together with earth fault protection. Further-
more, the DC MG short-circuit protection can be made with molded
case which includes DC circuit breakers that are more cost-efficient
than the fuses and over-current relay. On the AC side, the breaker
protects against power converter switching faults.
Therefore, by taking into account all the possibilities such as
operating states of every component connected within the given
MG system, technical selection for the MG operation and control,
MG operating mode and the microgrids fault ride-through
(MGs-FRT) requirements; a proper protection scheme design for
the microgrid systems can be achieved.
6. Conclusion and future trends
In this paper, the introspective review of the AC and DC MG
systems with renewable based DG units, energy storage devices
and loads that are available in recent literatures was presented.
Based on the growing energy demands and interests in generating
electricity from the renewable based DG units the need for
reliability, efficiency and quality power supply can finally be
achieved. The paper also described the problems associated with
the conventional distribution system and the roles of the MG
systems in the conventional energy systems, customer equipments
and the future energy systems. In addition, the survey on possible
DG units’ configuration in low voltage networks (LVAC and LVDC
networks), feasibility of MG systems, control strategies and their
protection techniques was discussed. Also, the influence of the MG
systems with the projected increase in the number of DC powered
components for residential and industrial application together
with more DG units that generate DC power reveals that the DC
MG systems will soon be the right candidates for the future energy
systems. On the other side, synchronization of the DG units, inrush
currents caused by transformers, induction motors and generators
and difficult in voltage control and system stability are still the
main challenges to investigate in the AC MG systems contrary to
the DC MG systems.
The general challenges remain on how to run every component
in MG systems at its optimal operating condition; to operate the
power converters while integrated with the smart grid technologies
(such as AMIs, measurements and sensing instruments etc.) and to
improve their capability to fault ride-through during grid distur-
bances. Again, the combined ESSs and renewable DG units provide
the user dispatch capability of its distributed energy resources.
Reliability and energy management technique of the ESSs are still a
hot research topic. On the downside, the power converters account
for a significant part of the total capital cost of a typical MG system
installation, and they are often the least reliable part in the whole
system design with low the short-circuit current limitation. There-
fore, from the commercialization perspective, the key business
needs for the power electronics devices are to reduce costs with
improved reliability and efficiency. Hence, the short-circuit current
limits require further investigations. It is also important for micro-
grids to have flexible protection schemes that can operate in both a
grid-connected mode as well as an islanded mode with proper relay
triggering currents while considering the operating state of every
component, mode of operation and the recommended microgrid
code for fault-ride through.
Another issue is related to information and communication
infrastructures (ICTs). A more centralized system control of micro-
grids requires a significant data flow towards a single central
point in order to achieve similar results. The problem becomes
extremely difficult and expensive to solve, if the real time
functionalities are required such as on line security assessment.
On top of that, the issue is the openness of the system which is still
a big challenge. Adopting a decentralized system, allows every
manufacturer of the DG units and ESSs or loads to embed a
programmable agent in the controller of his equipment according
to some rules. This provides the required “plug and play” cap-
ability of the future DG units, ESSs and loads. In opposite to the
centralized system, the installation of any new component would
require extra programming of the central controller.
Acknowledgments
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MEST)
(no. 2012R1A2A2A01045312).
References
[1] Asmus P. Microgrid, virtual power plants and our distributed energy future.
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J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405
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![6. Conclusion and future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
According to the history perspectives, AC power network has
been the standard choice for commercial energy systems to power
the incandescent lamps in our residences and motors in our
factories since the late of 19th century. The easy to transform AC
voltage into different levels for various applications, the capability
to transmit power over a long distance and its inherent character-
istic from the fossil energy driven rotating machine, made the AC
power network to become the main choice. To this point, the AC
power networks have existed for more than one century ago along
with the AC loads dominated in the market [1]. However, high
energy costs together with limited fund to construct new large
power plants and long distance transmission lines are some of the
constraints to meet the growing energy demands. Moreover, the
global warming, aging of the current power system infrastruc-
tures, increased awareness of limited energy generation resources,
higher energy consumption standards and the increased number
of DC loads due to advance in power electronics reveal that
modernization of the current energy system is inevitable [2].
In the last two decades, modern solutions such as renewable
based DG units, energy storage systems (ESSs), flexible AC trans-
mission systems (FACTS), active demand management (ADM), AC
microgrids and advanced control strategies based on information
and communication technologies (ICTs), have made possible for
energy engineers and researchers to redesign the conventional
power systems. However, not all these redesigning procedures are
accomplished until now, as more researches are needed to make
such advanced systems a reality. In that case, the designed and
implemented AC microgrid systems utilize the existing AC power
system standards such as frequency, voltage levels and principles
of protection for their operations [3,4]. On the other hand, the
introduction of grid control using the mercury vapor rectifier
around 1903 gave rise to electronic devices application in high
voltage direct current (HVDC) transmission in 1928. This was due
to their remarkable ability for rectification and inversion of DC
power [1,5]. Since then, the DC power systems had been restricted
only to some special applications such as telecommunication
systems, shipboards, tractions, HVDC transmission systems and
AC power systems for control and protection [6–8]. However, the
development in power electronics technology has raised a number
of DC loads and more power converters for DC voltage transfor-
mation into various levels for different applications [9–11]. Today,
the DC distribution systems (DC microgrid systems) are applied in
avionics, automotive, marine and manufacturing industries for
power distribution [12–14]. The international space station (ISS),
spacecraft, modern aircraft and modern electronics loads such as
computers and servers in data centers, and banks and electronics
factories, require DC power supplies [15–17].
To manage our future energy demands, a more configurable,
flexible, informative and physical energy system in the low voltage
distribution networks if not the whole power network, is manda-
tory [18–20]. Due to this point of view, microgrids (MGs) are
emerging and becoming more attractive structures with integra-
tion of renewable based DG units and ESSs [21,22]. So far, the
DG units include several technologies such as wind turbines
[23–25], photovoltaic (PV) arrays, fuel cells, and micro-turbines
[27] while the ESSs are the batteries, flywheels and super-
capacitors [28–30]. These components are the central ideas in
the microgrid structures and are also regarded as the solution to
the population without access to energy or with poor energy
supply and entitled to effectively reduce the greenhouse gas (GHG)
emissions [31–33]. Furthermore, the DG units provide a relief to
stressed conventional power transmission and distribution infra-
structures [34–36]. Besides, the liberalized electric market with
demands for efficient, reliable and diversified energy resources
constitutes the most important driving forces for high penetration
of DG units into the energy systems.
At present time, most of renewable based DG units directly
produce DC or variable frequency/voltage AC output power and
hence power electronics devices (PEDs) have become the key
elements in order to realize the MGs. Because of their vicinity to
different types of loads (i.e. AC and DC), the DG units such as the
fuel cells and the micro-turbines can be used to provide combined
heat and power (CHP) generation with improved overall efficiency.
With ESSs connected to the DG units such as fuel cells, a
controllable output power to meet the grid codes or load transient
requirements can be produced. The wind turbine and PV array
based DG units are usually controlled with maximum power point
tracking (MPPT) to maximize their energy generation [37–39].
Similarly, the ESSs with wind or PV based DG units can also
produce controllable output power to facilitate the dispatched
power generation and load demand matching [40–42]. Conse-
quently, the MG provides an opportunity to optimize the utiliza-
tion of renewable energy resources with improved overall thermal
and electrical efficiencies by properly locating different DG units
while considering their geographical conditions and the nature of
available loads [43]. Such operating conditions require the micro-
grid systems to have wide-range control systems in order to
perform large number of tasks [44]. For example, the guarantee
to system security, optimal operation, emission reduction and a
seamless transfer algorithm from grid-connected mode to islanded
mode without violating system constraints and regulatory require-
ments are some of the main tasks [45,46]. Also, the MGs for
standalone and grid-tied applications have been considered in the
past as separate cases [47–49]. However, it is important nowadays
to conceive flexible MGs that operate in both modes [50–52].
System protection within the microgrid components [53] and
short-circuit currents limitation can be achieved through a proper
coordination of the DG units [54–56].
This paper presents the latest researcher's literature review in
the field of AC and DC microgrid systems integrated with renew-
able based DG units, ESSs and mixed (AC and DC) loads. The
motivation to carry out this study is the growing interests of
renewable based DG units, ESSs and the continuous changing in
the type of customer's equipments from the dominated AC-type to
DC-type loads. Thus, a survey on microgrid systems with various
DG units configuration in low voltage AC (LVAC) and DC (LVDC)
networks in regard to their current and future applications in
customer equipments and energy market are included. Referring
to the economical, technical and environmental advantages of
renewable energy sources (RESs), a detailed overview between AC
and DC microgrids is provided in relation to their feasibility,
control strategies and protection approaches.
The paper is therefore structured as follows: a description of
the various DG units' configurations and their interconnections
with low voltage AC and DC networks in relation to the power
output is provided in Section 2. Study about the feasibility of
J.J. Justo et al. / Renewable and Sustainable Energy Reviews 24 (2013) 387–405
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